ALPHA-THROMBIN INHIBITS SIGNAL TRANSDUCERS AND ACTIVATORS OF TRANSCRIPTION-3 SIGNALING BY INTERLEUKIN-6, LEUKEMIA INHIBITORY FACTOR, AND CILIARY NEUROTROPHIC FACTOR IN CCL39 CELLS

We recently demonstrated that, in rat aortic smooth muscle cells, alph a-thrombin stimulated Stat3/SIF-A (signal transducers and activators o f transcription 3/sis-inducing factor-A) activity [G. J. Bhat et ab. ( 1997) Hypertension 29(Pt. 2), 356-360]. In the present study, we obser ved that exposure of CCL39 cells (a Chinese hamster lung fibroblast ce ll line) to alpha-thrombin resulted in a time-dependent decrease in ba sal SIF-A activity. We hypothesized that the decrease in basal SIF-A w as due to the initiation of an inhibitory pathway, following alpha-thr ombin exposure. To test this hypothesis, we determined if alpha-thromb in would inhibit Stat3 and SIF-A activation by interleukin-6 (IL-6), l eukemia inhibitory factor (LIF), and ciliary neurotrophic factor (CNTF ). In support of this hypothesis, alpha-thrombin inhibited the Stat3/S IF-A response induced by all the above cytokines. The inhibition by al pha-thrombin was concentration dependent, was sensitive to hirudin, an d. was mimicked by the thrombin receptor agonist peptide. The inhibiti on did not require the activation of phorbol 12-myristate 13-acetate-s ensitive isoforms of protein kinase C and was reversed by pretreatment with the mitogen-activated protein kinase kinase 1 (MAPKK1 or MEK1) i nhibitor PD98059. Inhibitory cross talk between alpha-thrombin and IL- 6 was also observed in MRC-5 cells, a fibroblast cell line derived fro m human lung tissue. Thus, we identify a novel alpha-thrombin inhibito ry pathway which, acting through a MAPKK1-dependent mechanism, blocks IL-6-, LIF-, and CNTF-induced Stat3/SIF-A activation. This inhibitory cross talk may provide an important regulatory function to modulate ge ne transcription by these cytokines, during immune and inflammatory re sponses. (C) 1998 Academic Press.